Briefly, a pressure-swing adsorption (PSA) plant comprises one or more beds of a material which has the ability to selectively adsorb a component (or components) of a gas mixture fed to it, to leave an unadsorbed gas which is rich in another component (or components) of the original feedstock. The operation of any one bed is necessarily cyclic, and a typical operating cycle for a bed where the `product` gas is the unadsorbed component(s) may comprise a first period in which the basic feedstock is made available to the bed, the undesired component(s) are adsorbed and the unadsorbed component(s) drawn off by means of a compressor; a second, regeneration period in which the product compressor is disconnected and a vacuum pump is applied to the `upstream` side of the bed to remove the adsorbed component(s); and a third period in which the bed is backfilled with product quality gas in preparation for the commencement of the next cycle. A typical PSA plant operating on this type of cycle may therefore comprise three adsorbent beds (sharing a common product compressor and vacuum pump) the operation of the beds being sequenced such that at any one time one bed is in the adsorption period of its cycle, another bed is in the regeneration period of its cycle and the remaining bed is in the backfilling period of its cycle. In this way a constant supply of product gas can be obtained. Two bed PSA plants using the same type of cycle are also known, the operation of the beds in such a plant being sequenced such that one bed is in its adsorption period while the other bed is in its regeneration and backfilling periods, and vice versa. Many variations in the arrangement of adsorbent beds and in the operating cycles are possible.
Synthetic zeolite materials are knwon which have sufficiently selective gas sorptivity to separate the two major components of air, i.e. nitrogen and oxygen, and pressure swing adsorption is becoming an increasingly important technique for the on-site production of oxygen and nitrogen from the atmosphere. One field in which the atmospheric PSA plant is particularly useful is sewage treatment, where there is a need to supply oxygen-rich gas to activated sludge reactors. Naturally, the demand for such gas at any particular time depends upon the quantity and condition of the material in the reactor, and can fluctuate quite considerably. It is therefore necessary to be able to match the gas supply to the demand (i.e. to achieve a useful turn-down range for the PSA plant).
One way to achieve the required matching of the gas supply to the demand is to keep the PSA plant operating at its rated capacity and simply to vent that amount of product gas which is excess to requirements. This is, however, very wasteful and results in the PSA plant consuming far more power than is necessary. The invention therefore primarily seeks to provide for a useful reduction in power consumption when the demand for product gas falls in the context of the supply of oxygen-enriched gas by a PSA plant as described above, bearing in mind that the major power consumer in such a plant is the vacuum pump; (when operating at the rated capacity of the plant the ratio of the power consumptions of the vacuum pump and product compressor is roughly equal to the ratio of the quantities of the adsorbed and unadsorbed components in the basic feedstock, or about 7:1 in the case of a PSA plant producing oxygen from the atmosphere). This is not to be taken to imply, however, that the invention is limited in its applicability to oxygen-producing PSA plants.